The present invention relates to addressing schemes for electronic displays, that is to say displays which use a display medium at least one optical characteristic of which can be varied by subjecting the medium to an electric field.
Electronic displays comprise a layer of electro-optic material, a term which is used herein in its conventional meaning in the art to refer to a material having first and second display states differing in at least one optical property, the material being changed from its first to its second display state by application of an electric field to the material. The optical property is typically color perceptible to the human eye, but may be another optical property, such as optical transmission, reflectance, luminescence or, in the case of displays intended for machine reading, pseudo-color in the sense of a change in reflectance of electromagnetic wavelengths outside the visible range.
One important sub-group of electronic displays are electrophoretic displays, which have been the subject of intense research and development for a number of years. Such displays can have attributes of good brightness and contrast, wide viewing angles, state bistability, and low power consumption when compared with liquid crystal displays. (The terms “bistable” and “bistability” are used herein in their conventional meaning in the art to refer to displays comprising display elements having first and second display states differing in at least one optical property, and such that after any given element has been driven, by means of an addressing pulse of finite duration, to assume either its first or second display state, after the addressing pulse has terminated, that state will persist without power for at least several times, for example at least four times, the minimum duration of the addressing pulse required to change the state of the display element.) Nevertheless, problems with the long-term image quality of these displays have prevented their widespread usage. For example, particles that make up electrophoretic displays tend to settle, resulting in inadequate service-life for these displays.
Numerous patents and applications assigned to or in the names of the Massachusetts Institute of Technology and E Ink Corporation have recently been published describing encapsulated electrophoretic media. Such encapsulated media comprise numerous small capsules, each of which itself comprises an internal phase containing electrophoretically-mobile particles suspended in a liquid suspension medium, and a capsule wall surrounding the internal phase. Typically, the capsules are themselves held within a polymeric binder to form a coherent layer positioned between two electrodes. Encapsulated media of this type are described, for example, in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426; 6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798; 6,177,921; 6,232,950; 6,241,921; 6,249,271; 6,252,564; 6,262,706; 6,262,833; 6,300,932; and 6,312,304, and in International Applications Publication Nos. WO 97/04398; WO 98/03896; WO 98/19208; WO 98/41898; WO 98/41899; WO 99/10769; WO 99/10768; WO 99/10767; WO 99/53373; WO 99/56171; WO 99/59101; WO 99/47970; WO 00/03349; WO 00/03291; WO 99/67678; WO 00/05704; WO 99/53371; WO 00/20921; WO 00/20922; WO 00/20923; WO 00/26761; WO 00/36465; WO 00/38000; WO 00/38001; WO 00/36560; WO 00/36666; WO 00/59625; WO 00/60410; WO 00/67110; WO 00/67327; WO 01/02899; WO 01/07691; WO 01/08242; WO 01/17029; WO 01/17040; and WO 01/80287.
An encapsulated, electrophoretic display typically does not suffer from the clustering and settling failure mode of traditional electrophoretic devices and provides further advantages, such as the ability to print or coat the display on a wide variety of flexible and rigid substrates. (Use of the word “printing” is intended to include all forms of printing and coating, including, but without limitation: pre-metered coatings such as patch die coating, slot or extrusion coating, slide or cascade coating, curtain coating; roll coating such as knife over roll coating, forward and reverse roll coating; gravure coating; dip coating; spray coating; meniscus coating; spin coating; brush coating; air knife coating; silk screen printing processes; electrostatic printing processes; thermal printing processes; ink jet printing processes; and other similar techniques.) Thus, the resulting display can be flexible. Further, because the display medium can be printed (using a variety of methods), the display itself can be made inexpensively.
It is shown in copending application Ser. No. 60/280,951, filed Apr. 2, 2001 that some particle-based electrophoretic are capable of gray scale and that such gray scale displays are stable not only in their extreme black and white states but also in their intermediate gray states. Methods to achieve gray states include spatial, temporal and voltage addressing methods such as dithering, time-width modulation, and voltage-height modulation. Thus, this type of display is properly called “multi-stable” rather than bistable, although the latter term may be used for convenience herein.
The bistable or multi-stable behavior of electrophoretic displays (and other electro-optic displays displaying similar behavior) is in marked contrast to that of conventional liquid crystal (“LC”) displays. Twisted nematic liquid crystals are not bi- or multi-stable but act as essentially instantaneous voltage transducers, so that applying a given electric field to a pixel of such a display produces a specific gray level at the pixel, regardless of the gray level previously present at the pixel. Furthermore, LC displays are only driven in one direction (from non-transmissive or “dark” to transmissive or “light”), the reverse transition from a lighter state to a darker one being effected by reducing or eliminating the electric field. Finally, a pixel of an LC display is not sensitive to the polarity of the electric field, only to its magnitude, and indeed to avoid undesirable effects, such as damage to electrodes, caused by current imbalance through a display, commercial LC displays usually reverse the polarity of the driving field at frequent intervals. In contrast, electrophoretic displays act as impulse transducers, so that the final state of a pixel depends not only upon the electric field applied and the time for which this field is applied, but also upon the state of the pixel prior to the application of the electric field.
The bistable or multi-stable (also sometimes referred to as “latching” or “memory”) characteristics of electrophoretic displays affect the choice of addressing methods to be used with such displays. Other factors which influence the choice of addressing methods to be used include the need for DC addressing because of the sensitivity of electrophoretic displays to the polarity of the electric field, and the desirability of DC-balanced waveforms; to avoid potentially harmful changes at the electrodes, it is desirable that the algebraic sum of the impulses applied to any given pixel of an electrophoretic display over time be zero. Also, the addressing method should allow for the lack of a threshold in electrophoretic displays; since such displays are impulse transducers, with a nearly linear relationship between (a) the product of voltage and time and (b) the change in optical characteristic of the display, even a small voltage applied for a substantial time will cause a change in the optical characteristic of the display. This normally precludes the use of passive matrix addressing methods.
Frequently, high resolution electrophoretic displays consist of pixels laid out in an array, with a transistor construction behind each pixel in a so-called “active matrix” arrangement. This arrangement can further be subdivided into sub-pixels. However all the transistors are in a plane, and control an array of bottom electrodes that are either on the same plane or on an adjacent plane just above or below. As such, the array is addressed in a two-dimensional or XY manner where pixels are referenced by row and column.
One problem with such high resolution displays is the sheer number of drivers required. In an array addressed in an XY manner, if there are x columns and y rows, x+y drivers are needed to address x*y pixels. For example, a color 640×480 display with three sub-pixels per pixel has 640×480×3=921,600 pixels and would typically be addressed by 480 row drivers and 1920 column drivers for a total of 2400 display drivers. Especially in small displays, the cost of the drivers rivals or may outweigh the cost of the electro-optic material.
In one aspect, the present invention provides an electronic display which reduces the number of drivers required.
Another problem with such high resolution displays is the time taken to address the display. Normally, in an active matrix display, one row of the display is activated at a time, and all the pixels in that row are addressed by applying appropriate voltages to the various columns. If addressing one row takes k seconds, at least k*y seconds are required to address the complete display. If the addressing scheme used requires that each row be addressed for the time necessary to completely address an electrophoretic medium (which is typically of the order of 100 milliseconds), the total time taken to address the display may be undesirably great. Similar considerations apply to displays using other types of electro-optic materials.
In another aspect the present invention provides various methods for reducing the time needed to completely address an electronic display.